Adhesive compositions used in conjunction with heat are old in the art. Heat sealing compositions have been used in the past to adhere a substrate to a surface or object, for example, one sheet of paper to another, a sheet of paper to a solid object, or a sheet of paper to a film, under the combined action of heat and pressure. However, heat-sealing compositions are not satisfactory for many adhesive uses, since in many applications it is difficult to simultaneously heat both the adhesive, the substrate and the object or surface to which the substrate is to be attached.
Heat-activated adhesive compositions have been developed which can be applied to a substrate in a thin non-tacky film, can be heat-activated and can remain activated or tacky for a period of time after activation. The advantage of heat-activated adhesives is that when heated the coating becomes tacky and remains tacky even after the coating cools. Large quantities of the substrate coated with the non-activated adhesive can be stored at room temperature in intimate contact without substantial adhesion, and release liners used with many tacky pressure-sensitive adhesives are unnecessary.
Heat-activated adhesive compositions are commonly applied to a substrate with high speed automatic machines. To maximize production, the films of adhesive must form rapidly and must dry quickly so the adhesive-substrate combination can be stored without adhesion. Further, the films must be formed in thin layers without void spaces lacking adhesive or localized excesses of adhesive.
The adhesive-substrate combination is commonly applied using rapid automatic machine application of the heated, activated adhesive-substrate combination to a surface or an object. The machine obtains the adhesive-substrate combination, heats it to activate the adhesive and rapidly applies it to the desired location. The adhesive must have reproducible heat activation properties and must activate quickly and uniformly. After heat activation, it is preferable that the pressure-sensitive adhesive have a controlled open time. In other words, the adhesive properties of the activated adhesive should remain present for a specific period of time. For certain applications, it is desired that the pressure-sensitive adhesive properties remain present for about 10 to 60 seconds or up to about 30 minutes, however for other applications it is important for the pressure-sensitive properties to remain for up to 8 to 10 hours or 72 to 96 hours.
In many applications of the adhesive-substrate combination such as the adhesion of price tags to goods in stores, it is desirable that the labels be essentially unremovable from packages. Packages of meats and other expensive commodities often have a price tag or label and the removal of the label and the substitution of another having a lower price is clearly undesired. If the strength of the adhesive bond resulted in the destruction of either the label or the package, the label could not be replaced. The destruction of the label can occur either causing the paper of the label to tear or causing a part of the film or object to which the label is applied to remain strongly attached to the label.